Steps

1

On Halo 1, a shotgun will take care of Combat Forms in the most expedient way possible (excepting a rocket launcher or any type of explosive). On Halo 3, duel wielding Plasma Rifles is your best bet. Any precision weapon will do as well, as long as you hit the Infection form that's embedded inside the Combat Form's chest. An almost just as good solution is to use the Magnum Pistol. It takes out most combat forms in 1 to 3 shots. Even on the harder difficulties such as heroic or legendary.

2

On Halo 3, there are Flood Pure Forms, which are a lot harder to kill. Tank Forms (the big ones) are nearly impervious to most weapons, however they will be killed instantly by an energy sword lunge, they can spew out infection forms as a special attack. Ranged Forms (the ones that cling on walls) will shoot at you from a distance. Stalker Forms (the little spider-looking things that crawl around you) will jump and attack you. Stalker Forms are the easiest to deal with. They are very weak. A simple melee will take care of them. use a sword (which are found on the bodies of elites or flood throughout the level) on the tank forms. on all difficulty settings they will die. for the ranged forms generally you will want to put them into shield mode and advance until you get within range to stick or melee them. one of each will kill them.

3

For Halo 3 The Flood can be generally be easily killed on any difficulty, the only time they put up a decent fight is when you are playing on Heroic difficulty, and they can be an actual challenge when you play on Legendary difficulty. Or if you turn any of the following Skulls; Mythic, Tilt, Famine, Fog, Thunderstorm, & Black-eye.

4

When you encounter a Tank Flood Form,UNSC weapons are weak against these huge things!If you find an Energy Sword or rocket launcher,and fuel rod cannon kill them quickly they can also pack a punch!

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Tips

For the love of Bungie smash the up the fallen bodies of the Flood Combat forms! If left intact another Infection form can skitter right over and take up the place where the previous one was inside of the Combat form and "bring it back to life". This happens more and more often on harder difficulties.

Don't try to shoot the Carrier Forms if they are close to you. Instead, wait until they are in a group of oncoming Flood. Kill it, and it will severely damage, if not outright kill, most of the other surrounding Flood forms.

Don't drive over the Infection Forms in Halo 3. They will explode and you will get knocked off course by the explosion.

If you notice a Carrier Form is nearby and about to explode, jump back and shoot it, the death animation can spare you more time from harm. (In case of wanting fun, jump up just as they explode and prepare to sail across the room)

Time your grenades. In Halo CE, they'll wipe out almost an entire hallway of Flood pretty quickly.

Warnings

In Halo 1, only engage in close-quarters combat with a Flood Combat Form if you have no other choice but to do so. They are very tough to kill; melees don't have much of an effect on them. Their melees are stronger than yours, and they tend to take out your shields on higher difficulties.

Don't use plasma on any Flood in Halo 1, as it is not very effective. In Halo 3, however, a Plasma Rifle is one of the best weapons against the Flood.

Do not use snipers on Flood! The bullet is too fast for them to feel the impact because their body is too soft. It'll go right through them.

Never turn you back on the fallen body of a Flood Combat form! It might just rise up again and beat you down for your stupidity.

Do NOT stick any Flood form with plasma grenades. They will run at you and take you out with them.